the actual flow velocity v may be calculated with the following formula:
v=Q/(A*n)=q/n, n is the porosity, and q the specific discharge

n is the porosity, and q the specific discharge. If the porosity n
is 30%, the flow velocity in the example above is 10.5 m/y

Student excercise: determination of the
hydraulic head of the 'groundwater model' (Fig)

establish a flow from left to right in the model

put dye into three wells tapping the upper (thick)
aquifer and measure the hydraulic head relative to the bench as a function
of distance from the recharge area, which is the left boundary of the sand
in the box.

after establishing a continuous flow in the model,
let the discharge flow into a (weighted) beaker for approximately 10 minutes.
Weigh the beaker afterwards and determine Q.

A is the average cross sectional area of the
sand perpendicular to the flow of the water, thickness * width

plot the hydraulic head as a function of distance,
and describe the function that you obtain

calculate k using Darcy's law by using the hydraulic
head measurements on both ends of the model

what simplifying assumptions are we making here?

Student excercise: determination of the
porosity of the top part of the 'groundwater model' (Fig)

inject some dye into one of the wells tapping
the sand on the left side of the model

while the model is running, determine the velocity
at which the plume is moving

use the above formula (n = q/v) to determine
the porosity

Aquifers ("bearing water")

aquifers: geologic formations that are porous and permeable
and yield significant amounts of water to springs or wells; aquitards,
aquicludes: formations that allow little flow (aquitards) or no flow
(aquicludes)

geology of aquifers (show examples)

unconsolidated sediments: loose granular deposit, particles are not
cemented together (e.g.: Long Island)

crystalline rocks: igneous and metamorphic rocks, e.g. Granite, have
often very low porosity, flow through fractures

porosities and hydraulic conductivities of different aquifer rocks

confined and unconfined aquifers

an unconfined aquifer has a water table (water table aquifer)

a confined aquifer does not have a water table. If you drill a well,
water will rise (in the well) above the top of the aquifer

perched groundwater is groundwater sitting on top of a poorly permeable
layer with an unconfined aquifer underneath

Storage of groundwater in aquifers

in many areas of the world the hydraulic head is declining with time
because a lot of water is pumped out of the aquifer, example from the southern
San Joaquin valley

storage in unconfined and confined aquifers is different

in unconfined aquifers the water pumped stems from drained void space

in confined aquifers the water stems from decompression of the water
and the sediments.

the same change in water table represents a larger amount of water
if taken from an unconfined aquifer as compared to a confined aquifer

definition of the storage coefficient:
yield per unit area and unit change in hydraulic head

unit: m3/m/m2 (-> dimensionless)

in unconfined aquifers the storage coeff. = porosity

in confined aquifer much smaller ~10-6

where is water being stored in confined aquifers?
-> compressibility of water and change in aquifer structure

land subsidence as a result of overpumping

examples:

the Dakota artesian basin: flowing artesian wells (hydraulic
head above surface) are wells in which the water level is higher than the
surface. A lot of wells were drilled into the Dakota basin, in South Dakota
about 15000 wells. Most of them do not flow anymore

New Mexico, where an old school well was still flowing when visited,
why did it break?

What information can be drawn from the hydraulic
head?

where the water is flowing (flownets, give examples)

how fast it is flowing

how much water there is -> storage coefficient

How to measure hydraulic head and hydraulic conductivity?

hydraulic head: install a well open to the aquifer
only over a small distance (short screen), measure the level of
the water in the well relative to a reference surface, for example sea
level